1. Field of the Invention
This invention relates to an optical tomography system for obtaining an optical tomographic image by measurement of OCT (optical coherence tomography).
2. Description of the Related Art
As a system for obtaining a tomographic image of an object of measurement in a body cavity, there has been known an ultrasonic tomography system. In addition to such an ultrasonic tomography system, there has been proposed an optical tomography system where an optical tomographic image is obtained on the basis of an interference of light by low coherence light. See, for instance, Japanese Unexamined Patent Publication No. 2003-172690. In the system disclosed in Japanese Unexamined Patent Publication No. 2003-172690, an optical tomographic image is obtained by measuring TD-OCT (time domain OCT) and the measuring light is guided into the body cavity by inserting a probe into the body cavity from the forceps port of an endoscope by way of a forceps channel.
More specifically, low coherence light emitted from a light source is divided into measuring light and reference light and the measuring light is projected onto the object of measurement, while the reflected light from the object of measurement is led to a combining means. The reference light is led to the combining means after its optical path length is changed. By the combining means, the reflected light and the reference light are superposed one on another, and interference light due to the superposition is detected by, for instance, heterodyne detection. In the TD-OCT measurement, a phenomenon that interference light is detected when the optical path of the measuring light conforms to the optical path of the reference light in length is used and the measuring position (the depth of measurement) in the object is changed by changing the optical path length of the reference light.
When measuring the OCT by inserting a probe into a body cavity, the probe is mounted on the system body to be demountable since disinfection, cleaning and the like of the probe after use are necessary. That is, a plurality of probes are prepared for one optical tomography system and the probes are changed by the measurement. However there is an individual difference in the length of the optical fiber due to the manufacturing errors and the like, and the optical path length of the measuring light can change each time the probe is changed. Accordingly, in Japanese Unexamined Patent Publication No. 2003-172690, on the basis of the reflected light from the inner surface of a tube (sheath) covering an optical fiber of the probe, the optical path length of the reference light is adjusted to conform to the optical path length of the measuring light.
Whereas, as a system for rapidly obtaining a tomographic image without sweeping the optical path length of the reference light such as disclosed in Japanese Unexamined Patent Publication No. 2003-172690, there have been proposed optical tomography systems of obtaining an optical tomographic image by spatially or time dividing the interference light (See, for instance, U.S. Pat. No. 5,565,986 or Japanese Unexamined Patent Publication No. 11(1999)-82817). Among those, a SD-OCT (source domain OCT) system where the frequency of light emitted from a light source is spatially divided to detect the interference light altogether has been proposed. In the SD-OCT system, a tomographic image is formed without scanning in the direction of depth, by emitting broad band, low coherence light from a light source by the use of a Michelson interferometer, dividing the low coherence light into measuring light and reference light and carrying out a Fourier analysis on each channeled spectrum obtained by decomposing the interference light of the reflected light, which returns when projecting the measuring light onto the object, and the reference light into frequency components.